True, but similarly, every kWh you charge from solar, you only get 0.9 back as well - so that 1 kWh of storage has cost me 12.4c worth of feed in - so I'm back to more than one six-pack. And, trust me, with a full household, an electric oven, dishwasher, A/C in summer, heater blower in winter, and peak that runs all the way through to 11pm, it's not hard at all to chew through most of 13.5kWh after sunset.
It's par 0.90 where it's PV or grid charged so you can eliminate that argument.
The math after that is whether PV and losing FiT is better to charge the battery with than grid, over the entire 10 year lifetime.
I tried doing 88 simulations by hand in NREL's System Advisor Model with different battery permutations of variables: lowest SoC allowed (0 to 80% in increments of 10%); charge before load; feed load before charging; charge via grid only on off-peak; charge via PV during TOU periods of 1, 1+2, 1+2+3, or 1+2+3+4, 2, 3, 4 (1=off-peak, 2=shoulder, 3=peak, 4=after-peak-shoulder). Rates are SCE's TOU-A.
Generally, charging off-peak via grid saves the most money, fastest payback on system-costs, and gives the highest net-present value just due to the lower base-cost.
Charging via PV and limiting discharge period to only the most expense TOU periods is next best but depth of discharge must be limited or it costs more to PV-recharge the battery next day.
For example, unlimited PV-charging before house-load, but discharging only in period 3, the best SoC is drain down to 10%, but saving a "$5053" out of $5050 annually. This can be adjusted to lowest SoC of 50% and still saving $5051 out of $5050.
Whereas, if feeding house-load before charging battery, it's again 10% as best lowest SoC, but saving a "$5094" out of $5050 annually. However, lowest SoC can be 70% and still saving "$5067" out of $5050.
Comparatively for grid-charging, discharging only at peak to max low of 10% saves "$5129" out of $5050 annually.
Free-discharging at periods 2,3,4 to max low of 10% SoC saves "$5834" out of $5050. Alternatively, to 80% SoC saves still "$5049" out of $5050 -- so grid-charging seems the best way to go to lower-the bill, while also preserving battery-life.